I have to write an essay for Heart of Darkness by Joseph Conrad. The question says in some works of literature a character who appears brifely or doesn't appear at all is a significant presence. Show how such a character functions in the work. You may wish to diiscuss how the character affects action, theme or the development of other characters. Advoid plot summary.

I choose to write about the doctor. He indicates that people will change internally once they go the Africa or any uncivilized country where there are no constraints of society, which is the theme. Why would Conrad include the doctor? Is there anything else I could add?